Quick-heating impregnated planar cathode and method of construction

ABSTRACT

A quick-heating planar cathode employs a disk of fine mesh wire of refractory metal around which is bonded or sintered a porous layer of tungsten particles impregnated with electron-emissive material, a layer of inorganic insulation covering one surface of the porous cathode, and a heater on the insulating surface.

United States Patent Beggs [111 3,792,513 [4 1 Feb. 19, 1974 QUICK-HEATING IMPREGNATED PLANAR CATHODE AND METHOD OF CONSTRUCTION Inventor: James E. Beggs, Schenectady, NY.

General Electric Company, Schenectady, NY.

Filed: Sept. 5, 1972 Appl. No.: 286,453

Related US. Application Data Division of Ser. No. 85,428, Oct. 30, 1970, Pat. No. 3,710,161.

US. Cl. 29/25.14 Int. Cl. H01j 9/02 Field of Search.... 29/25.l4; 313/346, 257, 337

Assignee:

References Cited UNITED STATES PATENTS 5/1970 Koppins 313/346 x 3,567,989 3/1971 Koshizuka 313/346 X Primary Examiner-Roy Lake Assistant Examiner.l. W. Davie Attorney, Agent, or F irmPau l I Edelson; Joseph [5 7] ABSTRACT A quick-heating planar cathode employs a disk of fine mesh wire of refractory metal around which is bonded or sintered a porous layer of tungsten particles impregnated with electron-emissive material, a layer of inorganic insulation covering one surface of the porous cathode, and a heater on the insulating surface.

4 Claims, 3 Drawing Figures QUICK-HEATING IMPREGNATED PLANAR CATHODE AND METHOD OF CONSTRUCTION This is a division of application Ser. No. 85,428, filed Oct. 30, 1970, now US. Pat. No. 3,710,161.

My invention relates to thermionic cathodes, and in particular to an impregnated porous planar cathode which may be quickly heated and its method of construction.

Tubes with quick-heating planar cathodes are required for many microwave applications and are extremely useful in many other instances, for example, in cathode ray tubes where rapid operation is desirable. Quick-heating cathodes currently available are of the type which employ a thin metal disk having one surface coated with electron-emissive material and having a heater wire bonded to its other surface and separated from that surface by a layer of insulation material. While this type of quick-heating cathode has been extremely useful, it is always subject to problems of separation of the heater from the metal disk because of shock. In contrast, impregnated cathodes, i.e., cathodes of porous structure, which are impregnated with electron-emissive material, are known to be more stable than oxide-coated cathodes under adverse operating conditions. However, a conventional impregnated cathode having a low mass suitable for rapid heating would be fragile and difficult to assemble.

It is the object of my invention to provide an impregnated cathode having a heating time comparable to that of quick-heating cathodes which are coated with electron emissive materials.

It is another object of my invention to provide a quick heating cathode structure which is reliable, has long life, and can endure shocks without separation of the heater from the cathode.

In its broadest aspects, my invention consists of providing a thin porous tungsten disk which is reinforced with fine wire mesh, the porous particles of the disk being sintered around the wire mesh. A layer of inorganic insulation covers one surface of the disk and a low-mass heater is deposited on the insulating layer. The porous tungsten disk is impregnated with electron emissive oxides, either before or after the insulation is applied.

The novel features believed to be characteristic of my invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood with reference to the attached drawing in which:

FIG. 1 is an exploded view showing certain components and steps in constructing a cathode embodying my invention;

FIG. 2 is a cross-sectional side view of a quickheating impregnated planar cathode of my invention, and

FIG. 3 is a partial cross section of a modification of the cathode of FIG. 2.

FIG. 1 illustrates several steps employed in my method of constucting a quick-heating impregnated planar cathode. A die 1 having a central recess 2 has deposited in that recess a thin layer of fine tungsten powder3. A disk, which may be either flat or cupshaped, as illustrated, of fine wire mesh 4, is inserted in recess 2 in contact with the tungsten powder particles and a plunger 5 is inserted within the wire mesh disk. Pressure is applied to the plunger to compact the powder particles and cause them to rigidly adhere to the wires of the mesh disk.

After removal from the die, the pressed disk with the particles adhering to it, is fired in hydrogen atmosphere at approximately 2,0002,500C to sinter the tungsten powder particles and bond them to the face of the cup or disk. After the composite disk is cooled, a thin layer of a suitable inorganic insulating material, such as, for example, boron nitride, is deposited, preferably by well-known chemical deposition processes, on the inside surface of mesh cup 4. A heater is then formed on the surface of layer 6 opposite the porous tungsten particles. Heater 7 may be formed using wellknown techniques, for example, by placing a mask bearing the desired heater pattern on layer 6 and vapor-depositing tungsten on layer 6 through openings in the mask. Alternatively, layer 6 may be completely covered, again using the vapor-deposition processes, with a layer of metal, such as tungsten, and portions of the layer not required for the heater are eroded, using an air gun to blast particles through a mask against the metal layer and form the heater.

The porous tungsten cathode may be impregnated with electron-emissive materials either before or after the step of depositing the boron nitride insulation. lf impregnation is performed before deposition of the insulating layer, preferably the composite structure of sintered tungsten powder particles and mesh is coated with the electron-emissive material and heated to a temperature of the order of 1,700C. During this step the oxides melt and completely permeate the porous tungsten structure. Thereafter, excess oxide material is removed from the surface, the cathode is heated to a temperature of the order of 1,200C and the insulating layer is applied by chemical vapor deposition.

In constructing a cathode of my invention, a two to three mil layer of tungsten powder was placed in recess 2 of a die 1 and a fine mesh wire selected from the group consisting of molybdenum, tungsten, and alloys of molybdenum and rhenium or tungsten and rhenium was formed into cup shape and placed in the die. The wires of the mesh were 1 mil thick. The mesh and tungsten powders were compressed to a total thickness of approximately two mils. When the pressed disk was removed from the die, the mesh was visible on one side only, while the powder was visible on the emitting side. After the structure had been sintered in a hydrogen furnace at a temperature of approximately 2,000C, the

' tungsten layer was covered with a suitable electronemissive material, such as barium calcium aluminate or barium calcium tungstate and heated to 1,700C to melt the coating material and cause it to penetrate the pores of the sintered structure. Excess material was cleaned from the surfaces. The cathode disk was then heated to a temperature of approximately 1,200C and a layer of boron nitride was chemically deposited on the upper surface of the cathode to a thickness of about one-quarter mil. Thereafter the heater was formed on the insulating surface, the heater having a thickness of approximately 0.2 mil, thus giving a total thickness of approximately 2.5 mils to the cathode. In use, such a cathode reaches a temperature of 1,000C in approximately 2 to 3 seconds.

FIG. 3 illustrates a modification of my cathode structure which is useful when certain of the materials employed may interact. In this cathode structure, a thin layer 8 of a barrier material, such as molybdenum, is

interposed between the composite disk comprising mesh 4 and tungsten particles 3 and insulating layer 6 to prevent or inhibit interaction between materials of the cathode and the layer of insulation.

From the foregoing it is apparent that l have provided a quick-heating impregnated planar cathode having all of the advantages of the impregnated type of cathode and whose heating time is comparable to that of conventional oxide coated cathode, without the limitations of the latter. The structure is rugged and, since there can be no separation of the heater from the cathode, assures reliable long life for the cathode.

-While the present invention has been described by reference to particular embodiments thereof, it will be understood that modifications may be made by those skilled in the art without actually departing from the invention. I, therefore, aim the appended claims to cover all such equivalent variations as come within the true spirit and scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. The method of constructing a quick-heating cathode which comprises pressing particles of tungsten powder into a fine refractory metal wire mesh, sintering the tungsten powder to bond it to the mesh, impregnating the sintered powder with electron-emissive material, depositing a layer of inorganic insulation on a surface of the sintered particles, and forming a heater on the layer of insulation.

2. The method of claim 1 in which wire ofa metal seganic insulation and the tungsten particles. 

2. The method of claim 1 in which wire of a metal selected from the group consisting of molybdenum, tungsten, an alloy of molybdenum and rhenium, and an alloy of tungsten and rhenium is used to form the mesh and boron nitride is vapor-deposited on the sintered tungsten particles.
 3. The method of claim 2 which includes the step of placing a mask on the insulating layer and vapor-depositing tungsten into the openings in the mask to form a heater.
 4. The method of claim 1 which includes the step of depositing a thin metal layer between the layer of inorganic insulation and the tungsten particles. 